Technically it is possible, because YUV frame has exact format necessary for JPEG encoding: the identical color model (YUV is almost YCbCr), plane format, IS_411 sampling model. But CJPEGEncoder::ColorConvert function, which is called for all encodings, will return UNSUPPORTED status, because there is no software path for this combination (source=IC_YCBCR, dest=IC_YCBCR, sampling=IS_411). You even don't need color conversion in this case, just to correctly set line steps for each of component planes in sources image structure. But, unfortunately there is no source code for that.